Recently the ultrasonic flowmeter for measuring a flow rate by counting a time necessary for an ultrasonic wave to pass through a propagation path and measuring a moving velocity of a fluid is utilized as a gas meter, and the like.
In the ultrasonic flowmeter, a pair of ultrasonic transmitter/receivers are provided to a pipe wall, along which a fluid (e.g., a gas) passes through, to intersect orthogonally with a flow of the fluid.
In case a flow rate of the fluid is measured by this ultrasonic flowmeter, first one ultrasonic transmitter/receiver emits an ultrasonic wave into a fluid, and then the ultrasonic transmitter/receiver receives the propagated ultrasonic wave and converts this ultrasonic wave into a voltage.
Then, the ultrasonic transmitter/receiver emits an ultrasonic wave into the fluid flowing through the inside of the pipe, and then one ultrasonic transmitter/receiver receives the propagated ultrasonic wave and converts this ultrasonic wave into a voltage.
Then, a flow rate of the fluid is detected based on the converted voltage, a velocity of the ultrasonic wave, an angle of an ultrasonic pulse in the propagation direction to the flowing direction of the fluid, a distance of a propagation path of the ultrasonic wave, etc.
Here, in a pair of ultrasonic transmitter/receivers, a matching member for transmitting the ultrasonic wave to the fluid in the passage is provided to an acoustic-wave emitting surface of a main body (transmitting/receiving surface of the ultrasonic wave) as an acoustic matching member respectively.
In the main body, a piezoelectric element is housed in a cylindrical case with bottom, an opening of the case is closed by a terminal plate, and terminals connected to the piezoelectric element via an electrically conductive rubber are supported by this terminal plate to protrude outwardly.
Also, the matching member has a matching layer made of a porous material (glass balloon (hollow)) to emit the ultrasonic wave into the fluid in the passage.
This matching member includes a first layer and a second layer whose acoustic impedances are different respectively, and is constructed such that the first layer is a composite material that is composed of a porous body and a filling material being bore in a porous hollow clearance, the second layer is formed of the filling material or the porous body, and no independent layer is present between the first layer and the second layer (see Patent Literature 1, for example).    Patent Literature 1: JP-A-2004-45389 Official Gazette